<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container{
width: 100px;
height: 100px;
background: red;
transition: width 1s, background 2s;
        }
       .container:hover{ /* （hover 尾类）当鼠标移到上面时*/
           width: 500px;
           background: blue;
       }
    </style>
</head>
<body>
    <div class="container"></div>
</body>
</html>